1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with improved reusable locking tubes for releasably locking the top nozzle on the upper ends of the control rod guide thimbles.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array of grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in passageways provided in the adapter plate of the top nozzle. The guide thimbles may each include an upper sleeve for attachment to the top nozzle.
During operation of such fuel assembly in a nuclear reactor, a few of the fuel rods may occasionally develop cracks along their lengths resulting primarily from internal stresses. Thus establising the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. Such products may also be released into a flooded reactor cavity during refueling operations or into the coolant circulated through pools where the spent fuel assemblies are stored. Since the fuel rods are part of the integral assembly of guide thimblies welded to the top and bottom nozzles, it is difficult to detect and remove the failed rods.
Until recently, to gain access to these rods it was necessary to remove the affected assembly from the nuclear reactor core and then break the welds which secure the nozzles to the guide thimbles. In so doing, the destructive action often renders the fuel assembly unfit for further use in the reactor because of the damage done to both the guide thimbles and the nozzle which prohibits rewelding.
In view of the high costs associated with replacing fuel assemblies, considerable interest has arisen in reconstitutable fuel assemblies in order to minimize operating and maintenance expenses. The general approach to making a fuel assembly reconstitutable is to provide it with a removable top nozzle. One reconstitutable fuel assembly construction, devised recently, is illustrated and described in the first U.S. patent application cross-referenced above. It incorporates an attaching structure for removably mounting the top nozzle on the upper ends of the control rod guide thimbles.
The attaching structure includes a plurality of outer sockets defined in an adapter plate of the top nozzle, a plurality of inner sockets with each formed on the upper end of one of the guide thimbles, and a plurality of removable locking tubes inserted in the inner sockets to maintain them in locking engagement with the outer sockets. Each other socket is in the form of a passageway through the adapter plate which has an annular groove. Each inner socket is in the form of a hollow upper end portion of the guide thimble having an annular bulge which seats in the annular groove when the guide thimble end portion is inserted in the adapter plate passageway. A plurality of elongated axial slots are provided in the guide thimble upper end portion to permit inward elastic collapse of the slotted portion so as to allow the larger bulge diameter to be inserted within and removed from the annular circumferential groove in the passageway of the adapter plate. In such manner, the inner socket of the guide thimble is inserted into and withdrawn from locking engagement with the outer socket.
The locking tube is inserted from above the top nozzle into a locking position in the hollow upper end portion of the guide thimble forming the inner socket. When inserted in its locking position, the locking tube retains the bulge of the inner socket in its expanded locking engagement with the annular groove and prevents the inner socket from being moved to a compressed releasing position in which it could be withdrawn from the outer socket. In such manner, the locking tubes maintain the inner sockets in locking engagement with the outer sockets, and thereby the attachment of the top nozzle on the upper ends of the guide thimbles.
Furthermore, to prevent inadvertent escape due to vibration forces and the like, heretofore the locking tubes have been secured in their locking positions. After insertion of the locking tubes into their locking positions within the inner sockets of the hollow upper end portions of the guide thimbles, a pair of bulges are formed in the upper portion of each locking tube. These bulges fit into the circumferential bulge in the upper end portion of the guide thimble and provide an interference fit therewith.
Another arrangement for securing a locking tube in its locking position within the upper end of the guide thimble and the passageway of the adapter plate is described and illustrated in French Pat. No. 2,529,704, wherein a number of crimps are formed between the locking ring or tube upper portion and the wall of the bore through the endpiece of adapter plate. Each crimp is composed of a "flat spot" on the wall of the locking ring or tube and an annular groove located facing and interacting with the flat spot. The annular groove is formed in the wall of the bore through the endpiece or adapter plate. The crimps are located above another larger concave annular groove in the adapter plate passageway which receives the circumferentially enlarged, slotted upper end portion of the guide thimble. It is believed that the crimps are formed by plastic deformation of the locking tube wall after the locking tube has been inserted into its locking position within the endpiece or adapter plate.
Then, when reconstitution of the fuel assembly is undertaken, these locking tubes must first be withdrawn by exerting axial force to overcome the interference fit. This partially collapses the locking tube bulges and "flat spots", having been earlier formed by plastic deformation, as the locking tubes are removed from the guide thimble upper end portions and the adapter plate passageways. After the failed fuel rods have been removed and replaced and following remounting of the top nozzle, the common practice is to discard the old locking tubes with the partially collapsed bulges and "flat spots" and install a full complement of new locking tubes on the guide thimble upper ends and secure them by plastic deformation.
This practice has a number of disadvantages. First, a large inventory of locking tubes must be maintained. Second, provision must be made for disposal of the discarded irradiated locking tubes. Third, after each locking tube is inserted, a deforming operation must be carried out remotely to produce the bulges in each tube. And, fourth, an inspection of bulges must be carried out remotely to ascertain whether the bulges were made to the correct dimensions. Consequently, notwithstanding the overall acceptability of the use of the above-described attaching structure in reconstitutable fuel assemblies, these recently recognized disadvantages have created a need for further improvement of the reconstitution operation so as to enhance commercial acceptance thereof.